1. Technical Field of the Invention
The present invention relates generally to a pedestrian-vehicle collision detecting apparatus, and more particularly to a simple and power-saving structure of such a pedestrian-vehicle collision detecting apparatus.
2. Background Art
There have been proposed automotive pedestrian impact guard systems which have a vehicle-pedestrian collision detector installed in a front bumper of the vehicle. The vehicle-pedestrian collision detector works to detect an accidental collision with a pedestrian during traveling of the vehicle and trigger deployment of an airbag mounted on an upper surface of the front of the vehicle or lift up the hood of the vehicle to absorb physical impact acting on, especially the head of the pedestrian falling onto the upper surface of the front of the vehicle.
For example, Japanese Patent First Publication Nos. 8-216826 and 11-310095 disclose the above type of vehicle-pedestrian collision detectors. The detector, as taught in the former publication, has an impact sensor made up of conductive rubber plate in which metallic fine particles are mixed and electrodes affixed to the conductive rubber plate. The impact sensor is installed over a length of the front bumper of the vehicle and works to detect impact with a pedestrian correctly even if the pedestrian hits any portion of the front bumper. The detector, as taught in the latter publication, has a pressure sensor equipped with an elastic tube filled with gas and works to sense occurrence of a collision with the pedestrian upon elevation in internal pressure of the elastic tube.
The above vehicle-pedestrian collision detectors, however, have a difficulty in distinguishing between impacts with pedestrians and other sorts of impacts. In order to avoid this problem, Japanese Patent First Publication No. 11-028994 proposes use of a combination of a collision impact (or collision-caused deformation of a sensor), an impact duration, and a vehicle speed. Japanese Patent First Publication No. 11-310095 proposes use of a combination of a collision-caused deformation of a sensor, a change in such deformation with time, and a vehicle speed. Specifically, systems, as taught in these publication, are designed to use a typical phenomenon that when hit by the vehicle, pedestrians are usually struck up from the vehicle, that is, that immediately after such a collision, legs of the pedestrian are kicked up by the bumper of the vehicle, so that after reaching a peak subsequent to the collision, the magnitude of impact load sensed by the sensor or the degree of deformation of the sensor attenuates.
The inventors of this application have found in the above systems that a period of time until the magnitude of impact load or degree of deformation exceeds a given threshold value (will referred to as a collision duration below) depends upon a positional relation between right and left legs of pedestrians upon collision with the vehicle.
Usually, when the vehicle is traveling at approximately 40 km/h, a period of time between when only one leg of the pedestrian is hit by the bumper and a time when the leg is struck or kicked up by the bumper, or a period of time between when both legs of the pedestrian (which are aligned with a travel direction of the vehicle) hit the bumper and a time when the legs are kicked up by the bumper are on the order of 10 to 20ms. In the latter case, when kicked up by the bumper, one of the pedestrian's legs in contact with the bumper strikes the other leg. The collision duration is, thus, substantially identical with that in the former case.
However, in most cases, both legs of pedestrians are not arrayed parallel to the bumper upon collision with the vehicle, so that the collision duration depends greatly upon distances between one of the legs and the bumper and between the other leg and the bumper. The time when the above described magnitude of impact load or the degree of deformation decreases below the threshold value (i.e., the end of the collision duration) is, therefore, much later than that when only one of the legs or the legs aligned to the travel direction of the vehicle are kicked up by the bumper.
The collision duration in a case where a pedestrian is walking perpendicular to a travel direction of the vehicle, and legs of the pedestrian are not aligned with the travel direction of the vehicle upon a hit against the vehicle will be described below with reference to FIGS. 1 and 2.
FIG. 1 illustrates a positional relation between legs of a pedestrian and a motor vehicle immediately before the pedestrian is hit by the vehicle. The right leg L1 is closer to the bumper of the vehicle (i.e., a line sensor installed on the front of the vehicle) than the left leg L2 by about 150 mm. In this example, the collision duration ΣT for which the sensor continues to sense impact with the pedestrian is, as shown in FIG. 2, given substantially by the sum of a right leg-collision duration ΔT1 and a left leg-collision duration ΔT2 and approximately two times longer than that when only one of the legs L1 and L2 is hit by the vehicle. For instance, when the vehicle is traveling at 40 km/h, the collision duration will be more than 35 msec, thus resulting in a great increase in time required to discriminating between impacts with pedestrians and other types of impacts based on the attenuation of the above described impact load or the deformation.
However, if an adult of an average physical size is struck by the vehicle, his or her head usually hits the hood of the vehicle approximately 120 ms after collision when the vehicle is traveling at 40 km/h, thus consuming much time in discriminating between an impact with the pedestrian and other types of impacts. The hood or the airbag must, therefore, be lifted up or deployed completely in a very short space of time, which requires the need for enhancing the performance of the system. This results in increases in size of the system and/or electric power consumed by the system, thus leading to an increase in manufacturing cost and a difficulty in mounting the system on motor vehicles.